Hans Peter Broedel
Together with generally good weather and the rapid clearing of arable land, the new modes of farming improved crop yields, leading to rising population. During the high middle ages, Europe’s population doubled, from about 37 million to 74 million, with much of this increase concentrated in the North and West: modern Britain, France, the Netherlands, and Germany.1 The most productive medieval manors were the most specialized; they farmed the most productive crops most intensively. These manors, however, had to trade for everything they could not supply themselves, and required markets in which to sell their surplus. They needed towns. For this reason, although there were only 29 towns with more than 5000 inhabitants in Europe before 1000 CE, by 1200 there were 127.2 Towns generated wealth: they produced cloth, tools, glass and metal ware, furniture, and luxury goods; they provided markets for the sale of grain, fruit, and animals; and they housed butchers, brewers, and specialists of many kinds.
Townsfolk needed very different sorts of technologies than did rural farmers, and they eagerly adopted tools of all sorts when they became available. This happened more frequently than in the early middle ages because the growth of towns and trade in Europe was part of a more general rise in population, agricultural yields and trade that extended all the way from Sung China to West Africa.3 As Europeans became increasingly engaged in this emerging global economy, they gained access to a host of technologies developed in other cultures, and proved adept at adapting them to their own particular needs. One development particularly important for the future was the effort to replace the work of humans and animals with alternative sources of power, a trend particularly noticeable in the spread of mills. Mills are technically machines for grinding or cutting, but when we talk of medieval mills we usually are talking more generally about machines that convert the work of humans, animals, wind or water into rotary power, most commonly to grind grain, but also for many other purposes. Most medieval rivers and streams boasted at least a few mills, and in some places they were extremely common. They were built for the usual reason: they were profitable. In return for an admittedly significant capital investment, mill owners could exploit a resource that was basically free instead of relying on power sources that had to be fed, or, worse, paid. Manor lords were particularly fond of watermills, because lords were able to require their tenants to pay to use them; peasants caught illicitly grinding their own grain at home were liable to pay heavy fines.4
Watermills, of course, had been in use for centuries, but growing wealth encouraged their spread; and more remarkable were the simultaneous efforts to use waterpower in other industries. A key enabling invention was the cam, an irregularly shaped wheel or cylinder that, when mounted on a shaft, can conv